Nmr has been shown to be an extremely powerful technique for determining the chemical environments and structure of an inhibitor or substrate bound to an enzyme. These studies involve observing the effect added enzyme has on the nmr line shapes, chemical shifts, and relaxation times of the signals of a small molecule which is undergoing chemical exchange between the free solution and the enzyme active site. We have used this nmr chemical exchange method to probe the structure of the 3'-CMP, 2'-CMP, and 5'-CMP complexes with bovine pancreatic RNase A. A study of the 'H nmr of the 1', 5, and 6 protons and the P31 Nmr of the inhibitors has shown that each inhibitor binds quite differently to the Rnase A active site. This detailed information about the mode and kinetics of binding of these inhibitors is applied to our evolving understanding of the mechanism of action of RNase A.